1. Field of the Invention
The present invention relates generally to the removal of organic materials on various substrates, and, more particularly, to an ashing method for removing organic films and materials temporarily formed on various substrate layers during fabrication of semiconductor, flat panel display, read/write heads, and other related devices.
2. Description of the Related Art
Removal of the photoresist film is an important part of the process of fabricating semiconductor devices. The use of ashing methods, in particular, using a gas with high oxygen content, for removing organic films, such as resists and polyimides has been known for some time. The advances in plasma tools and the related processing techniques, over the last decade, have managed to keep up with the challenges of successive generations of Very Large Scale Integration (VLSI) and Ultra Large Scale Integration (ULSI) devices. However, as the size of the features and the thickness of films in these devices continue to decrease, the manufacturing challenges are also renewed with every generation of Integrated Circuits (ICs).
As the dramatic shrinking of IC geometries continues, the ashing methods are continuously faced with two problems: (a) achieving higher rates of residual-free resist removal and (b) lowering the amount of damage caused in the substrate layers underlying the resist film. These generally conflicting objectives are addressed by changing either the physical conditions of the plasma medium or the chemical conditions of the ashing process. For example, one can achieve higher rates of processing by either generating a dense plasma environment or by using or generating, in the plasma environment, chemical species that react more efficiently with the resist.
Substrate damage can likewise be attributed to both physical and chemical conditions of the plasma. For example, charging and ion bombardment effects are directly related to the physical properties of the plasma. Energetic ions can drive small quantities of heavy metal (i.e., Fe, Cu and Pb) and alkaline metal (i.e., Na and K) atoms, which are generally present as impurities in the resist films, into the substrate layer underneath the resist. The heavy metal contamination and in particular the subsequent permeation and migration of heavy metals into other substrates (e.g. silicon) layers can affect the minority carrier lifetime to the detriment of the device properties. Such bombardment effects become more severe as the resist films become thinner towards the end of the ashing process, particularly as the thickness of sensitive substrates are designed to be thinner.
Substrate damage also results from the chemical properties of plasma, such as etching or other poisonous effects on the layer underneath the resist. For example, etching of silicon oxide (SiO.sub.2) occurs because of fluorine (F), when halogenated gas mixtures such as oxygen (O.sub.2) and tetrafluoromethane (CF.sub.4) are used to increase the rate of plasma ashing. Similarly, energetic oxygen ions can contribute to the formation water inside the surface layers of spin-on-glass (SOG) films, resulting in an increase in the dielectric constant or in the related via-poisoning phenomenon.
These considerations apply, to various degrees depending on the application, to all conventional dry-etch plasma etchers, e.g., barrel, down-stream or parallel-electrode configurations, with the down-stream ashing being the most widely used method. To increase the processing rates and minimize the problem of ion damage, techniques for higher plasma densities and lower ion energies may be employed. The new generations of advanced plasma sources achieve these objectives by decoupling the control of the plasma density from the control of ion energy in the plasma by such techniques as Electronic Cyclotron Resonance (ECR) or Inductively Coupled Plasma (ICP) in microwave or radio frequency power regimes. The art of these and other types of plasma technologies and plasma tools are well known and have been the subject of many US patents.
Independent of the nature and the regime of the plasma employed, the rate and completeness of ashing as well as any unwanted etching or damage to the substrate layer, in the conventional ashing tools, are strongly influenced by the chemical reactions between the resist and the substrate layer and the reactive ionic, neutral and radical species generated in the plasma. In a typical down-stream or other conventional asher, the nature of the plasma gas mixture is the primary determinant of the ashing rate which is also sensitive to the "ashing temperature". The nature of the gas mixture also influences the activation energy of ashing which is a measure of the sensitivity of the ashing rate to the ashing temperature.
The activation energy is obtained from the gradient of the Arrhenius plot which is a line plot of the ashing rate as a function of the inverse ashing temperatures. Therefore, a small activation energy (small slope of the Arrhenius plot) indicates that ashing rate is less sensitive to ashing temperature, and that the ashing process will be more stable and uniform. Lower activation energies also imply that the ashing temperature can be lowered without significant loss of ashing rate. This is particularly useful where VLSI or ULSI fabrication requires lower processing temperatures and yet where acceptable practical levels of ashing rates (i.e., &gt;0.5 .mu.m/min) must be maintained.
A thorough discussion of ashing rates and activation energies for a series of gas mixtures consisting of one or more of the following oxygen, hydrogen, nitrogen, water vapor and halogenide gases is given in the U.S. Pat. No. 4,961,820. It is shown that addition of nitrogen to oxygen plasma does not change the activation energy (0.52 eV for oxygen) and improves the rate of ashing only slightly (from 0.1 to 0.2 .mu.m/min at 160.degree. C.). However, addition of 5 to 10% hydrogen or water vapor to oxygen reduces the activation energy to about 0.4 eV with a similar improvement in the ashing rate as in the case of nitrogen addition. Addition of both nitrogen and 5 to 10% of either hydrogen or water vapor to oxygen plasma has a synergistic effect of increasing the ashing rate to a more practical level of 0.5 .mu.m/min (at 160.degree. C.).
The most dramatic improvements in the activation energy (down to 0.1 eV) and the ashing rate (&gt;1.5 .mu.m/min) are obtained when a halogenide (e.g., tetrafluoromethane) is added to the oxygen plasma. However, in this case, CF.sub.4 also results in etching of such substrate layers as silicon oxide, polysilicon and aluminum due to fluorine reaction. It is reported that inclusion of water vapor in the reactant gas mixture will reduce the damage by CF.sub.4 apparently as a result of the reaction of water with CF.sub.4, thus suppressing the halogen action.
As seen from the above discussion, the search for a satisfactory reactant gas mixture, with reasonably high ashing rate and without any deleterious effect on the substrate layer underneath the resist film, continues. Furthermore, as the constraints of the VLSI and ULSI fabrication become more stringent, lower ashing temperatures and ashing-process stability (lower activation energy) increasingly become major requirements of a satisfactory reactant gas mixture.